The objectives of this project are the determination of the nature and biological properties of rickettsial antigens and constituents and the development of procedures for classification of spotted fever group rickettsiae. Hybridomas producing monoclonal antibodies were prepared from mice infected with Rickettsia rickettsii. Of the 31 monoclonal antibodies thus far tested for immunoglobulin subclass, 11 belonged to the IgG2A sublclass, nine to the IgG2B subclass, and seven to the IgG3 subclass. Four did not react with any of the isotyping sera. Immunoblotting tests showed that five of the antibodies complexed with epitopes present on molecules of various sizes; these molecules may be polysaccharide in part since the ability of the determinants to combine with antibody was not affected by treatment with the proteolytic enzyme proteinase K. With "standard" radioimmune precipitation tests, it was found that 20 monoclonal antibodies reacted with a 170,000-dalton antigen, and six monoclonal antibodies precipitated both 133,000- and 32,000-dalton molecules. Only those antibodies specific for the 170,000- and 133,000-dalton antigens proteced mice from a lethal challenge with R. rickettsii. Antibodies to the latter antigens were detected in mouse-protective sera from human patients convalescing from Rocky Mountain spotted fever. Immunoelectron microscopy revealed that both protective and unprotective monoclonal antibodies reacted with antigens on the rickettsial surface. An attempt will be made to purify and characterize the 133,000- and 170,000-dalton antigens.